A large proportion of congenital heart defects are seen in the valves and membranous septa of the heart, which are in part derived from the endocardial cushions. The morphogenesis of the endocardial cushions is a complex process, involving a sequence of basic developmental events. These include cell migration, proliferation, and differentiation. It is the goal of this project to define this developmental sequence in greater detail and to understand the role of specific genes in the process. The major focus is on components of the extracellular matrix. The following questions will be addressed. l) What is the role of the homeobox gene, Hox 7, in the initiation of cushion morphogenesis? This will be addressed by examining the effects of anti-sense deoxyoligonucleotides on cushion morphogenesis and on the expression of extracellular matrix components in heart explant cultures and in ovo. 2) What extracellular matrix molecules are expressed during cushion formation? This will be addressed by immunohistochemistry with a battery of antibodies available in the laboratory and by in situ hybridization in some cases of special interest. 3) What is the developmental effect of blocking the expression of specific extracellular matrix components on cushion morphogenesis? This will be carried out with blocking antibodies, where available, or anti-sense deoxyoligonucleotides. In particular we will focus on hyaluronan and its binding proteins (CD-44 and PG-M), collagen II, and a component of the cardiac jelly (1E12) which has been identified recently in this laboratory. 4) The role of other genes in cushion morphogenesis will be studied subsequently, as they become characterized. 5) What is the contribution of the cushion cells to the tissues and extracellular matrix of the mature valves and septa of the heart? This question will be addressed by immunohistological analysis after in situ labeling of cushion cells with replication-defective, lac-Z labeled retrovirus, followed by the reincubation of the windowed, injected chicken eggs.